It is known that, by construction, an aircraft may not be perfectly symmetric with respect to its vertical mid-plane and that it exhibits a lateral dissymmetry which, although small and of no danger to safety, presents drawbacks as will be seen hereinafter. Moreover, it is known that such a lateral dissymmetry of construction is manifested in flight mainly by a roll-wise dissymmetry, related to the considerable wingspan.
Such roll-wise dissymmetry can be compensated by differential adjustment of aerodynamic surfaces of the aircraft, such as the high-lift flaps, or by installing a deflector (“wedge”) on one of the wings. However, these methods of compensation present several drawbacks:                for it to be possible to correct the dissymmetry, it must firstly be measured in flight, then, after application of the modification, the correction must also be checked in flight: these methods therefore demand flight time;        these methods also demand fine tuning on aircraft after leaving the production chain;        the correction applied to aircraft is definitive: if for one reason or another, the dissymmetry alters over time (modification of the loading or of the layout, works on the airfoil, etc.), then the correction applied is no longer suitable;        if the dissymmetry noted in flight is not of aerodynamic origin, then an aerodynamically symmetric aircraft is still aerodynamically dissymmetric after correction.        
Since, in an obvious manner, the roll-wise dissymmetry of an aircraft is manifested, in flight, by a lateral inclination of the craft, it is possible, in order to avoid the drawbacks of the known methods recalled above, to envisage controlling the ailerons so as to cancel said lateral inclination. Nevertheless, in this case, said ailerons are deflected with respect to their aerodynamically neutral position, so that they generate some increase in the drag and the performance of the aircraft is degraded. Thus, it is then necessary to choose between passenger comfort (zero lateral inclination) and aircraft performance.